Finally a document on HTTP.sys!
Just got a ping from the http.sys team alerting me to this link where you'll find a paper on http.sys on Vista and Longhorn.
http://www.microsoft.com/downloads/details.aspx?FamilyID=311f4be8-9983-4ab0-9685-f1bfec1e7d62&displaylang=en
-bret